1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly but not exclusively to integrated circuit packaging.
2. Description of the Background Art
Semiconductor integrated circuits dissipate power during operation, referred to by the term “Pd.” In the past, the majority of integrated circuits had Pd's of less than 2 Watts. Some types of integrated circuits (e.g. microprocessors) had Pd's greater than 2 Watts. These integrated circuits require packaging capable of efficiently dissipating power in order to keep the junction temperature of the integrated circuit within acceptable limits. Such packaging is referred to as “thermally enhanced.” There are many techniques to thermally enhance a package. A common theme of these conventional techniques is to provide high thermally conductive paths from the silicon die to the outside of the package where the power is dissipated into the printed circuit board (PCB) on which the integrated circuit is mounted and/or into the air above the top surface of the integrated circuit. Each conventional thermal enhancement tends to add additional cost to the package. In general, greater performance results in higher cost. Generally speaking, the Pd of integrated circuits has increased with advancements in integrated circuit technology (in terms of the number of features and feature size) to the point that many integrated circuits now require some form of thermal enhancement. However, many of these integrated circuits cannot afford the additional costs associated with thermal enhancement. Thus, there is a need for a thermally enhanced package having a cost close to that of existing standard (non-thermally enhanced) packages, while offering significant thermal performance.
FIG. 1 schematically shows an example Plastic Ball Grid Array (PBGA) package 100 with thermal balls 102. In the PBGA package 100, the die 101 is mounted on the topside of the substrate 105 and covered by a molded cap 104. The substrate 105 is designed with a section of ground plane directly under the die 101 to which it is bonded. This ground plane connects to a similar ground plane on the bottom side of the substrate 105 using an array of thermal vias 103. An array of thermal solder balls 102 is attached to this bottom side ground plane. The thermal vias 103 provide a high thermal conductivity path from the backside of the die 101 through the substrate 105 to the thermal solder balls 102, which then dissipate heat to the PCB on which the PBGA package 100 is mounted. The PBGA package 100 provides enhanced thermal performance when compared to a PBGA package without thermal vias.
FIG. 2 schematically shows an example Heat Sink Ball Grid Array (HSBGA) package 200 with thermal balls 102 coupled to thermal vias 103. The HSBGA package 200 is very similar to the PBGA package 100 except for a metal heat sink 202 embedded in the mold cap 104 above the die 101. The heat sink 202 has tabs that extend down to the topside surface of the substrate 105 where they contact metal traces. These traces are generally connected to a metal area located under the die 101. This allows the traces to conduct heat away from the die 101 to the heat sink tabs, which then conduct the heat to the exposed metal on the topside of the package. This second path for heat conduction to the topside of the package improves thermal performance.
FIG. 3 schematically shows an example L2-Ball Grid Array (L2-BGA) package 300. The L2-BGA package 300 is quite different from the previously discussed packages 100 and 200. In the L2-BGA package 300, the die 301 is mounted upside down in a cavity on the bottom side of the package 300. This is commonly referred to as a “cavity down” package. The die 301 is bonded within the cavity to a flat metal heat spreader 302. The substrate has a cut-out area to accommodate the die 301, which is fitted to the bottom of the heat spreader 302. Mold compound is used to cover the die 301. In the L2-BGA package 300, there is a very direct path for heat to flow from the back-side of the die 301 to the heat spreader 302 and the topside of the package. The L2-BGA package 300 has the best thermal performance compared to packages 100, 200, and 500 (see FIG. 5).
The L2-BGA package 300 operates in the following manner. There are three methods of heat transfer: conduction, convection and radiation. Of the three, conduction is the most effective. At first glance, it appears that the L2-BGA package 300 with its topside heat spreader 302 is designed to enhance thermal performance by providing convective and radiative cooling from the top surface. However, the L2-BGA package 300 still transfers up to 90% of its heat by conduction through its solder balls 303 to the PCB on which the package is mounted. Thus, the heat spreader 302 actually serves to spread heat from the hot central area of the package (around the die) to the whole surface of the package, thereby allowing, more heat to travel through the solder balls 303 at the edges of the package to the PCB.
A second feature of the L2-BGA package 300 is that the topside metal heat spreader 302 provides an excellent place to mount an external heat sink 402 as shown in FIG. 4. The heat sink 402 together with forced airflow greatly increases heat transfer through convection. It is common for very high power integrated circuits (e.g. those with Pd greater than 6 Watts) to be packaged in an L2-BGA package, but require that the customer attaches an external heat sink. The factory does not ship the integrated circuits with heat sinks attached because customers generally want to optimize the heat sink for their particular application.
To summarize, the two essential features of the L2-BGA package 300 are the ability to spread heat across the whole surface of the package and the capability to attach an external heat sink to the top of the package.
FIG. 5 schematically shows an example CSBGA package 500. The CSBGA package 500 is a low cost reduced thermally enhanced ball grid array (BGA) package. In the CSBGA package 500, the die 301 is mounted within a cavity of the substrate 502. The CSBGA package 500 is very similar to the L2-BGA 300 except that the CSBGA package 500 uses a standard 4-layer substrate 502 and it dispenses with the stiffener ring. Both of these features reduce the cost of the starting material compared to an L2-BGA package without greatly impacting thermal performance.
Examples of PBGA, HSBGA, L2-BGA, and CSBGA packages are also available from ASE Inc. (Internet URL: <http://asegloabal.com>).
The above mentioned packages have several disadvantages. Generally speaking, the PBGA and HSBGA packages only work well for integrated circuits that dissipate less than about 3 Watts of power. The L2-BGA package is typically used with integrated circuits with power dissipation greater than 3 Watts. However, the L2 BGA package has two major disadvantages.
The first disadvantage of an L2-BGA package has to do with its starting material, which comprises a combined substrate and heat sink. The substrate usually comprises 3-layers and utilizes laser drilled blind vias. This is a more expensive technology compared to a standard 4-layer substrate with thru-hole vias. In addition, an extra manufacturing step is required to create the center cavity to accommodate the die. The heat sink is actually manufactured from two separate pieces of nickel-plated copper sheet. The top piece is the main heat spreader. The bottom piece has a hole punched out to accommodate the die and acts as a stiffener ring. Both pieces are bonded together with a layer of fiberglass and epoxy resin. This complex assembly is needed to control warping of the package caused by the mismatch of thermal coefficients of expansion of the various different materials used. This heat sink assembly is then bonded to the substrate. This multi-layer assembly is substantially more expensive when compared to a simple 4-layer substrate such as that used in a PBGA package.
The second disadvantage of an L2-BGA package relates to its manufacturability. The L2 BGA package is assembled as individual pieces. This makes it less efficient to manufacture compared to packages that can be assembled in strip form. The L2-BGA package is offered in a number of standard body sizes, for example 27×27 mm, 31×31 mm, and 35×35 mm. For each different body size, a different set of manufacturing tooling is required to handle the package during manufacture. Additionally, because each package is a single piece, each one must be handled separately. In comparison, other packages (e.g. PBGA) are manufactured in a strip of multiple packages. The strips have a common form factor, allowing the use of a single set of manufacturing tools to manufacture many different sizes and configurations of the basic package type. Furthermore, as each strip contains multiple packages (e.g. eight packages) and the manufacturing process needs to only handle strips, there is far less handling on a per-package basis. These factors result in the manufacturing cost of the L2-BGA package being higher due to additional capital, and also result in variable costs to be higher due to increased handling.
A CSBGA package attempts to reduce the cost of an L2-BGA package in two ways. First, the CSBGA package uses standard 4-layer substrate technology. Second, the CSBGA package uses a simpler heat spreader. However, the CSBGA package still suffers from the manufacturability problems of the L2-BGA package. Furthermore, in practice, the CSBGA package is not significantly cheaper than the L2-BGA.
What is needed is a low cost packaging alternative that is capable of spreading heat and readily allows for the use of an external heat sink.